In turbines, in particular in turbine stages of turbines, turbine parts, such as inner walls or platforms, are exposed to hot working fluid of the turbine. Hence, cooling systems are used for cooling the turbine parts.
For cooling purposes, cooling air systems are installed for providing a cooling air stream to the hot turbine parts and/or preventing the hot stream of combustion gases to escape past the inner and outer walls of the main flow annulus. The space for installing a cooling air system and for providing an effective cooling circulation of the cooling fluid is limited. Furthermore, complex cooling systems are expensive and the reliability is low. Moreover, a high amount of cooling air my affect negatively the turbine performance.
Moreover, in a gas turbine, axial gaps, required for assembly and thermal expansion during operation between the various gas path components such as the inner casing elements, may allow ingress of hot gas into the cavities outside of the gas paths boundary. The cavities are generally formed between inner casing elements and supporting structures for the inner casing elements.
Hot gas ingress is undesirable because it leads to the requirement for expensive high-grade temperature resistant materials for the components of the turbine that are in contact with the hot gas outside the main flow annulus. In order to reduce the temperature, a high amount of purge air (cooling air) is then required to prevent or limit the hot gas ingestion, which typically exceeds the need for cooling purposes of the walls defining the main flow annulus. The amount of purge air may be significant due to the size of the outer volume itself but also due to the presence of circumferential pressure variations in the main fluid direction of the hot gas in an inner volume of the turbine, which causes higher ingress. The circumferential pressure variations in the working fluid are caused by the vanes inside the inner volume of the turbine. The high amount of purge air leads generally to an engine performance loss, both as a lower specific power output and reduced cycle efficiency. In some engine designs, the availability of coolant supply pressure and or in combination with coolant mass flow may be limited which gives potential for hot gas ingress and or further performance losses as a coolant of higher pressure will have to be used.
In conventional approaches, the cavities which are formed between inner wall elements, inner casing elements and outer casing elements are purged with sufficient compressed air to reduce and to eliminate hot gas ingress or reduce it to an acceptable level.
U.S. Pat. No. 6,402,466 B1 discloses a leaf seal for gas turbine stator shrouds and a nozzle band. A leaf seal assembly is secured to a trailing edge of a shroud segment for sealing between the shroud segment and the leading edge sidewall of a nozzle outer band. The leaf seals include a circumferentially elongated seal plate biased by a pair of spring clips disposed in a groove along the trailing edge of the shroud segment to maintain the seal plate in engagement with the flange on the leading edge sidewall of the nozzle outer band.